support tooltip for systemd-failed-units

[teto]

I would like to format the systemd-failed-units such that it shows a boolean "No error"/"errors" and have the actual number in a tooltip.

For instance:

	"systemd-failed-units": {
		"hide-on-ok": false,
		"format": "✗",
		"format-ok": "✓",
		"system": true,
		"user": true,
		"tooltip": true,
        "tooltip-format": " {nr_failed} failed Systemd units",
	},

man waybar-systemd-failed-units doesn't shoow the "tooltip" but I thought this would be a "default" attrtibute supported across all modules so I enabled it and waybar doesn't error but still when I hover the module, it doesn't show any tooltip while it shows for other tooltips such as the clock.

waybar --version Waybar v0.10.3